1. Field of the Invention
The present invention relates generally to a temperature sensor and method for ring temperature. More particularly, the present invention relates to a low-power micro-mechanical temperature sensor using a micromechanical thermal bimorph structure.
2. Background of the Invention
Embedding miniature sensors in products, systems, storage and shipping containers, and other items allows the monitoring of those items to determine health, maintenance needs, lifetime, and other item characteristics. Information from miniature temperature sensors can tell a user whether the item has been exposed to temperature levels that can cause damage. In addition, miniature temperature sensors can be used to “wake up,” from a low-power sleep mode, a more sophisticated sensing system to collect a more complete set of environmental data.
Current battery-powered embedded sensor systems that perform this type of monitoring often require a low power method of determining when a certain level of temperature has been reached. Many other applications, such as in transportation and shipping monitoring, heating and air conditioning, and food storage, would benefit from the ability to monitor the temperature environment with a completely unpowered sensor. In addition, these applications would benefit from the ability to poll that sensor to determine if a temperature extreme was reached, and then reset the sensor for later use. In either case, an ultra-low power sensor, or even a sensor that consumes no quiescent power, would reduce the overall system power consumption enough to allow embedded sensors to operate for decades in portable battery powered applications, or in systems that scavenge small amounts of power from the environment.
Low power and unpowered temperature sensors currently exist. However, they are large-scale devices such as bimetallic strips in standard thermostats. These devices operate in a somewhat similar fashion (albeit on a much larger scale) and provide a somewhat similar function as the present invention, but are not in a form factor suitable for integration with microdevices, and are not fabricated using techniques that are compatible with microelectronics or micro-electromechanical systems (“MEMS”) devices.
Micro-scale temperature sensors exist, as well, but most of the previous work to develop low-power temperature sensors has been focused on minimizing the power consumption of standard miniature devices, and using low-power analog electronics to determine when a specific temperature level has been reached. These devices and systems would then create a low-impedance logic level signal for input to a sleeping microcontroller. The fundamental problem is that such a system must continuously power the temperature sensor and analog trigger circuitry, creating a constant power draw on the batteries. Even using the latest in low-power devices and highest capacity batteries, systems that continuously power any sensor will only operate for 5-10 years.
As embedded miniature sensors get smaller, and as batteries are reduced in size and capacity, the use of lower power and unpowered devices will become more critical. Furthermore, maximizing the sensor functionality, without increasing power consumption, will enhance the capability of embedded sensing systems.
Other inventions have used thermally actuated micro-machined devices to measure temperature, and for switching, but none have had the advantages of the present invention in combining low- or no-power operation with a mechanical latching function.
For example, U.S. Pat. No. 5,917,226, entitled “Integrated Released Beam, Thermo-mechanical Sensor for Sensing Temperature Variations and Associated Methods,” provides an integrated circuit for sensing temperature variations using a thermal bimorph device, but the sensor does not have a latching function and does not operate without quiescent power.
U.S. Pat. No. 4,916,349, entitled “Latching Piezoelectric Relay,” utilizes a thermal bimorph in a latching piezoelectric relay. This invention requires the application of external power to cause the relay to latch and has a latching function that is piezoelectric and not mechanical.
U.S. Pat. No. 5,463,233, entitled “Micromachined Thermal Switch,” utilizes a thermal bimorph device as a temperature-activated switch, but this invention does not have a latching capability.